Line speed dependent control of a furnace for heat treating aluminum alloy sheet

ABSTRACT

A method for controlling continuous heat treating and annealing of heat-treatable and non-heat-treatable aluminum alloy sheet at final thickness continuously moving in a floating state horizontally through a continuous convection floating furnace arranged to heat the moving aluminum sheet to a set peak metal temperature. The controlling including controlling fan speeds and furnace air temperature to accommodate variations in line speed of the aluminum alloy sheet continuously moving in the floating state horizontally through the continuous convection floating furnace

FIELD OF THE INVENTION

The invention relates to a method for controlling continuousheat-treating and annealing of aluminum alloy sheet at final thicknesscontinuously moving in a floating state substantially horizontallythrough a continuous convection floating furnace arranged to heat themoving aluminum sheet to a set peak metal temperature (T_(PMT)). Thecontrolling including controlling fan speeds and furnace air temperatureto accommodate variations in line speed of the aluminum alloy sheetcontinuously moving in the floating state horizontally through thecontinuous convection floating furnace.

BACKGROUND TO THE INVENTION

As will be appreciated herein below, except as otherwise indicated,aluminum alloy designations and temper designations refer to theAluminum Association designations in Aluminum Standards and Data and theTeal Sheets Registration Record Series as published by the AluminumAssociation in 2018 and frequently updated, and well known to thepersons skilled in the art.

For any description of alloy compositions or preferred alloycompositions, all references to percentages are by weight percent unlessotherwise indicated.

In the production of motor vehicles in particular aluminum alloys theAA5000- and AA6000-series alloys like 5051, 5182, 5454, 5754, 6009,6016, 6022, and 6111, and various others, have been used to produceautomotive structural parts and body-in-white (“BIW”) parts.

The industrial scale automotive sheet production of the heat-treatableAlMgSi-alloy series, also known as 6000-series aluminum alloys, typicalexamples include AA6005, AA6014, AA6016 and AA6022, comprises severaldiscrete steps. A rolling slab or ingot is subjected to semi-continuousdirect chill (DC)-casting or electromagnetic casting (EMC-casting), alsocontinuous casting like belt or roll casting can be applied. The rollingslab or ingot may be preheated at about 500° C. to 580° C. for severalhours for homogenization of the microstructure. Then the rolling slab oringot is hot rolled into hot rolled strip at a gauge of about 3 to 12mm, the hot rolled strip is typically hot coiled and cooled down toambient temperature. The hot rolled strip is cold rolled to final gaugein several passes, optionally an intermediate anneal is applied prior tothe cold rolling or during the cold rolling process, and at final gaugethe strip is annealed to adjust the required material properties. Thesolution heat treating can be done either in a continuous heat treatingfurnace or in a batch type furnace.

An economical attractive method of producing 6000-series aluminum sheetis by means of continuous solution heat treating at final gauge. At theend of a continuous solution heat treating furnace, the strip materialis rapidly cooled or quenched to ambient temperature, for example bymeans of forced air cooling or spray cooling systems. By this solutionheat treating the main alloying elements Mg and Si are dissolved,leading to a good formability, control of the yield strength and bakehardening behaviour, and brings the sheet material to a T4 temper.

7000-series aluminum alloys are heat treatable aluminum alloyscontaining zinc as the predominate alloying ingredient other thanaluminum. For purposes of the present application, 7000-series aluminumalloys are aluminum alloys having at least 2.0% Zn, and up to 10% Zn,with the zinc being the predominate alloying element other thanaluminum.

International patent application WO-2010/049445-A1 (Aleris) discloses astructural automotive component made from an aluminum alloy sheetproduct having a gauge in a range of 0.5 to 4 mm, and having acomposition consisting of, in wt. %: Zn 5.0-7.0%, Mg 1.5-2.3%, Cu max.0.20%, Zr 0.05-0.25%, optionally Mn and/or Cr, Ti max. 0.15%, Fe max.0.4%, Si max. 0.3%, and balance is made by impurities and aluminum. Thesheet product has been solution heat treated (“SHT”) and cooled,artificially aged, after aging formed in a shaping operation to obtain astructural automotive component of predetermined shape, and subsequentlyassembled with one or more other metal parts to form an assembly forminga motor vehicle component, and subjected a paint-bake cycle.

Continuous annealing comprises continuously moving uncoiled nonheat-treatable aluminum alloy sheet, for example AA5000 series aluminumsheets, in the direction of its length through a continuous annealingfurnace and subsequently quenching the sheets after exiting the furnace.

Continuous solution heat treating also comprises continuously movinguncoiled heat-treatable aluminum alloy sheet, for example AA6000 sheetor AA7000 sheet, through a continuous heat treating furnace andsubsequently quenching the sheets after exiting the furnace. Aftersolution heat treatment, the alloy sheet can be hardened at roomtemperature (e.g., naturally aged) for a duration, hardened for aduration at a slightly elevated temperature (e.g., artificially aged orpre-aged), and/or otherwise further processed (e.g., cleaned,pretreated, coated, or otherwise processed).

Published US patent application no. 20170253953 to Meyer et al disclosescontinuously heat treating by moving heat-treatable AA6000-seriesaluminum alloy sheet substantially horizontally through a convectionfloating furnace.

Published US patent application no. 20170306466 to Meyer et al disclosescontinuously moving heat-treatable AA7000-series aluminum alloy sheetsubstantially horizontally through a convection floating furnace.

To provide longer production runs for continuous annealing or continuoussolution heat treating of aluminum sheets the aluminum sheets areattached end to end in series and fed to the furnace. Typically, a firstcoil of the aluminum sheet is unrolled to form a sheet and fed to thefurnace to be processed and a subsequent coil of the aluminum sheet isunrolled to form the next sheet to be processed. Then the leading end ofthis second sheet is attached in series to the trailing end of theprevious sheet. Thus, a continuous aluminum sheet is fed to the furnace.The trailing edge of the sheet being processed is typically stopped toconnect this trailing edge to the leading edge of the new sheet to beprocessed to form a joint.

When used for continuous annealing the continuous heat treatment linesof the invention anneal the aluminum sheet so it is important to controlpeak metal temperature.

Solution heat treatment is similar to annealing, but it involvesquenching, which is the rapid cooling of the alloy to preserve thedistribution of the elements. When used for solution heat treating thecontinuous heat treatment lines heat the alloy to a temperature at whicha particular constituent will enter into solid solution followed bycooling (quenching) at a rate fast enough to prevent the dissolvedconstituent from precipitating. Maintaining the desired peak metaltemperature during heating in both of these processes is important toobtain a product having the desired properties.

Annealing and solution heat-treatment involve heating and cooling thesheet to specific temperatures and holding at those temperatures forspecific durations of time. The temperature-time profile of sheet cangreatly affect the resulting strength and ductility of the final sheetproduct. In a continuous annealing line as well as a continuous solutionheat treating line deviation of nominal line speed leads to processconditions in the heat treatment furnace and quench that might lead tonon-conforming product characteristics.

Thus, temperature control of the set (target) peak metal temperature ina furnace for both annealing and solution heat treating is desired to bewith a control accuracy of +/−3° C. or better.

For purposes of this specification, peak metal temperature (“PMT”) isthe highest temperature that aluminum alloy achieves in the furnace of acontinuous annealing line or a continuous solution heat treating line.

Thus, the furnace line needs to continue to run sheet through thefurnace during deviation of nominal line speed to maintain the sheetbeing processed at the desired peak metal temperature for the desiredtime. If the sheet being processed merely stops in the furnace with nochanges to heating by the furnace during the attaching then the sheetwould overheat. Thus, a first accumulator or looper upstream of thefurnace provides a first buffer portion of the sheet to feed through thefurnace to continue advancing a downstream portion of the aluminum sheetthrough the furnace while the attaching is being performed. However,this first buffer resource is finite.

Also, after heat treating and quenching the sheet exiting from quenchingis cut by shears (for example flying shears) to separate a product sheetfrom the remainder of the sheet upstream of the cut. The portion of thesheet at the location of the cut is typically stopped during cutting.However, the furnace line needs to continue to run sheet through thefurnace during this attaching to maintain the sheet being processed atthe desired peak metal temperature. Thus, a second accumulator or looperdownstream of the furnace provides sufficient space to accumulate asecond buffer portion of the sheet received from quenching to continueadvancing an upstream portion of the sheet through the furnace while thecutting is being performed. However, this second buffer resource isfinite.

The buffers permit the sheet to continue moving through the furnace atits desired line speed. As a result, the target peak metal temperaturefor the target duration of the target peak metal temperature in thefurnace can be maintained during the time permitted by the first and/orsecond buffer.

However, if it is determined that the stoppage upstream or downstream ofthe furnace will occur for an extended time beyond that permitted forfeeding from the first buffer or accumulating in the second buffer atthe target line speed then the line speed must be reduced to extend thetime for the first and/or second buffer to operate. Once the accumulatoris empty (entrance) or full (exit) process section stoppage isunavoidable. Indeed, in addition to time expansion caused by issues atentrance and exit, there can be other reasons why a reduced line speedis desired to provide additional time or reduce scrap length, such asthe following:

Quality: e.g. dents: time to find and remove the source

Logistics: e.g. next coil not available or change of schedule

Reducing line speed can harm the sheet product properties because thetarget PMT and time duration of the target PMT in the furnace are notmaintained.

Thus, if a portion of the line upstream or downstream of the furnaceneeds to be stopped for longer than can be compensated by the firstand/or second buffer it would be beneficial to be able to control peakmetal temperature during this time, while the line in the furnace slowsdown to maintain conforming consistent product properties.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for continuouslyheat treating or annealing aluminum alloy sheet at final thickness toproduce sheet that has consistent good mechanical properties.

It is another object of the invention to control peak metal temperature(PMT) in a heat treating furnace, which is a continuous convectionfloating furnace, within +/−5° C., preferably +/−3° C., more preferably+/−2° C. of a target peak metal temperature by adjusting line speed, fanspeeds, and furnace air temperature in at least one zone of the furnacewhen a portion of the sheet feeding the furnace or a portion of thesheet exiting the furnace is slowed down for an extended period of time.

It is another object of the invention to control peak metal temperature(PMT) in a continuous furnace that can typically perform 1) annealing ofnon-heat treatable AA5000-series alloy; 2) solution heat treating ofAA6000-series alloy; or 3) solution heat-treating aluminum AA7000-seriesalloy sheet.

This and other objects and further advantages are met or exceeded by thepresent invention providing a method for continuously heating aluminumalloy sheet at final thickness in a continuous heat-treating furnacehaving an entry section and an exit section, wherein the heat treatingfurnace is a continuous convection floating furnace, comprising:

continuously horizontally moving uncoiled aluminum alloy sheet in afloating state in a path along a direction of its length through aplurality of contiguous heat treatment zones of an elongated heattreatment chamber of the continuous heat-treating furnace arranged toheat the moving aluminum sheet to a set peak metal temperature (T_(PMT))in the temperature range of 350 to 590° C.;

wherein the contiguous heat treatment zones have independentlycontrollable convection heaters along the path for heating the aluminumalloy sheet and independently controllable fans blowing above and belowthe aluminum alloy sheet along the path for guiding the aluminum alloysheet along the path as the aluminum alloy sheet horizontally movesthrough the elongated heat treatment chamber,

wherein at least one said contiguous heat treatment zone is a peak metaltemperature zone which has a target aluminum alloy sheet temperaturewhich is the peak metal temperature of the aluminum alloy sheet in theelongated heat treatment chamber;

taking measurements representative of heat transfer to the aluminumalloy sheet as the aluminum alloy sheet moves through the elongated heattreatment chamber, the measurements including speed of the aluminumalloy sheet through the elongated heat treatment chamber, speed of thefans blowing above and below the aluminum alloy sheet, and furnace airtemperature, and optionally surface temperature of the sheet,

wherein said taking measurements comprises:

continuously measuring the line speed of horizontal movement of thealuminum alloy sheet through the furnace and generating a speed signalproportional to the actual measured line speed v_(line,act) of thealuminum alloy sheet through the furnace, wherein the line speed of thealuminum alloy sheet through the furnace has at a line speed set point(v_(line,Set)),

continuously measuring the speeds of the furnace fans above and belowthe aluminum alloy sheet in said peak metal temperature zone, whereinthe fan speeds of the fans above and below the aluminum alloy sheet haverespective fan speed set points (v_(fan,Set)), wherein the fan speed setpoint of the fans above the aluminum alloy sheet may be the same ordifferent from the fan speed set point of the fans below the aluminumalloy sheet;

continuously measuring the furnace air temperature in said peak metaltemperature zone, wherein the furnace air temperature in said peak metaltemperature zone has a zone air temperature set point (T_(zone,Set));and

optionally continuously measuring the surface temperature of the sheet,

wherein during normal operation the actual measured line speed, measuredfan speeds in said peak metal temperature zone, and measured furnace airtemperature in said peak metal temperature zone simultaneously arerespectively in the preset ranges for the line speed set point(v_(line,Set)), the fan speed set points (v_(fan,Set)), and the zone airtemperature set point (T_(zone,Set)) in said peak metal temperaturezone; changing the fan speeds and the furnace air temperature in saidpeak metal temperature zone, in response to the actual measured linespeed v_(line,act) and the line speed set point v_(line,Set) accordingto equation (I) and equation (II):

$\begin{matrix}{v_{{fan},{act}} = {\left( \frac{v_{{line},{act}}}{v_{{line},{Set}}} \right)^{x} \times v_{{fan},{Set}}}} & (I) \\{T_{{zone},{act}} = {\left( \frac{v_{{line},{act}}}{v_{{line},{Set}}} \right)^{y} \times T_{{zone},{Set}}}} & ({II})\end{matrix}$

wherein:v_(line,act) is the actual measured line speed, for example in m/min,v_(line,Set) is the line speed set point, for example in m/min,v_(fan,act) is the actual fan speed in rpm in the peak metal temperaturezone,x is between 0.6 and 0.95, preferably 0.65 to 0.95, more preferably 0.75to 0.95,v_(fan,Set) is the fan speed set point in rpm in the peak metaltemperature zone,wherein the set point of each fan above the moving sheet may be the sameor different from the set point of each fan below the moving sheet,T_(zone,act) is the actual zone air temperature in the peak metaltemperature zone,T_(zone,Set) is the zone air temperature set point in the peak metaltemperature zone, y is between 0 and 0.2, preferably 0 and 0.1.

In the invention, the zone air temperature set point (T_(zone,Set)) fora respective zone lies within a respective preset temperature range. Theline speed set point (v_(line,Set)) lies within a preset range for thespeed of the aluminum alloy sheet. The respective fan speed set points(v_(fan,Set)) lie within respective preset fan speed ranges. The zoneair temperature set point (T_(zone,Set)) for a respective zone lieswithin a respective preset temperature range.

Generally, furnace air temperature at normal undisrupted operation isabove peak metal temperature to provide the driving force for heattransfer to heat the moving sheet. Thus, in normal operation the sheetachieves a peak metal temperature initially in a designated zone andmaintains temperature in a range from the peak metal temperature to thesoaking temperature T_(Soak) which is the predetermined desired minimumtemperature selected for annealing or solution heat treating, insubsequent downstream zones until it is cooled after exiting thefurnace. By definition T_(Soak) is lower than peak metal temperature(T_(PMT)).

However, when line speed slows in the zone where peak metal temperatureis initially achieved, and likewise the other zones of the furnace, theinvention reduces the fan speed and reduces the furnace air temperature.Reducing fan speed while still keeping the moving aluminum sheetfloating provide less hot air to the nozzles blowing hot air on thesheet to reduce convection heating of the advancing sheet. Reducing thefurnace air temperature to a temperature closer to the peak metaltemperature is achieved by admitting outside cooling air and/or reducingheating from the burner. This reducing of furnace air temperaturereduces the temperature driving force to heat the advancing sheet. As aresult, at the slower line speed the sheet will initially arrive at thedesired peak metal temperature in the same zone as it initially didduring normal operation or in an earlier upstream zone. In either case,approximately the same peak metal temperature is achieved but it is heldfor a longer soak time. If it continues to be initially achieved in thesame zone the soak time is longer because the sheet is moving slower. Ifit is achieved upstream of the zone in which it normally initiallyachieves peak metal temperature the soak time is longer not only becauseit is moving slower but also because the sheet is moving through morezones at peak metal temperature.

The control of the peak metal temperature by fan speed and furnace airtemperature compensation maintains temperature typically within +/−5°C., preferably +/−2° C. of target peak metal temperature for alldecelerating, constant, or accelerating speed conditions.

The invention may control the sheet temperature by fan speed and furnaceair temperature compensation, not only in one or more peak metaltemperature zones, but also in additional zones of the furnace. Thus,taking measurements in the method of the invention may further comprise:

continuously measuring the speeds of the furnace fans above and belowthe aluminum alloy sheet in one or more additional zones of saidcontiguous zones, said additional zones being in addition to said peakmetal temperature zone, wherein for each additional zone the fan speedsof the fans above and below the aluminum alloy sheet have respective fanspeed set points (v_(fan,Set)), wherein the fan speed set point of eachfan above the aluminum alloy sheet may be the same or different from thefan speed set point of each fan below the aluminum alloy sheet;

continuously measuring the furnace air temperature in each additionalzone, wherein the furnace air temperature in each additional zone has arespective zone air temperature set point (T_(zone,Set)); and

optionally continuously measuring the surface temperature of the sheet,

wherein during normal operation the actual measured line speed, measuredfan speeds in each additional zone, and measured furnace air temperaturein each additional zone simultaneously are respectively in the presetranges for the line speed set point (v_(line,Set)), the fan speed setpoints (v_(fan,Set)), and the zone air temperature set point(T_(zone,Set)) in the additional zone;

respectively changing the fan speeds and the furnace air temperature ineach additional zone, in response to the actual measured line speedv_(line,act) and the line speed set point v_(line,Set) according toequation (I) and equation (II):

$\begin{matrix}{v_{{fan},{act}} = {\left( \frac{v_{{line},{act}}}{v_{{line},{Set}}} \right)^{x} \times v_{{fan},{Set}}}} & (I) \\{T_{{zone},{act}} = {\left( \frac{v_{{line},{act}}}{v_{{line},{Set}}} \right)^{y} \times T_{{zone},{Set}}}} & ({II})\end{matrix}$

wherein:v_(line,act) is the actual measured line speed, for example in m/min,v_(line,Set) is the line speed set point, for example in m/min,v_(fan,act) is the actual fan speed in rpm in the additional zone,x is between 0.6 and 0.95, preferably 0.65 to 0.95, more preferably 0.75to 0.95,v_(fan,Set) is the fan speed set point in rpm in the additional zone,wherein the set point of the fans above the moving sheet may be the sameor different from the set point of the fans below the moving sheet,T_(zone,act) is the actual zone air temperature in the respectiveadditional zone,T_(zone,Set) is the zone air temperature set point in the respectiveadditional zone,y is between 0 and 0.2, preferably 0 and 0.1.

In the invention, the continuous measuring of the surface temperature ofthe sheet may be accomplished by an infrared temperature gun or othertype of pyrometer, for instance, in the peak metal temperature zoneand/or one or more additional zones. The invention may employ a feedbackloop with sheet surface temperature measurement(s) instead of, or inaddition to, using a model to determine the metal temperature, e.g.,peak metal temperature, for example a computer model based on monitoringand measuring fan speed and zone furnace air temperature to determinethe heat transfer rate along with line speed measurement to determinethe time the metal is exposed to those conditions.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic representation of the method and the apparatusused.

FIG. 2 shows a schematic drawing of the first looper accumulator.

FIG. 3 shows a schematic drawing of a zone of the continuous convectionfloating furnace.

FIG. 4 schematically shows a portion of the upper nozzle header box toillustrate nozzles that discharge into the space within the elongatedheat treatment chamber of the furnace.

FIG. 5 schematically shows a portion of the lower nozzle header box toillustrate nozzles that discharge into the space within the elongatedheat treatment chamber of the furnace.

FIG. 6 shows data from an example.

DESCRIPTION OF THE INVENTION

FIG. 1 provides a schematic representation of the method in accordancewith the invention and the continuous heat-treatment furnace used. Thecontinuous heat-treatment furnace (1) is arranged to transport and toheat-treat uncoiled aluminum sheet (2) moving in the direction of itslength along direction of travel “T”. The aluminum sheet is uncoiledfrom coil (8). Typically, the aluminum alloy sheet (2) at final gaugehas a thickness in the range of 0.3 mm to 4.5 mm, preferably of 0.7 mmto 4.5 mm. The sheet width is typically in the range of about 700 mm to2700 mm.

FIG. 1 shows the aluminum sheet (2) moving through a first looperaccumulator (12) upstream of the furnace (3). FIG. 1 also shows a joiner(16) upstream of looper 12 and a shearing station (18) downstream ofsecond looper accumulator (14). The joiner (16) attaches a leading endof the roll (8) to the trailing end of the sheet (2). For example,joining may be by welding, e.g., by means of friction stir welding.

Then the moving aluminum sheet (2) passes within the detection range ofa line speed sensor (13) which detects the speed of the moving aluminumsheet (2) in its direction of travel “T”.

Then the moving aluminum sheet (2) is gradually heated up from roomtemperature (RT) to the set peak metal temperature (T_(PMT)) as it movesthrough the elongated heat treatment chamber (3) of the continuousheat-treatment furnace (1) having an entry portion (4) and an exitportion (5). The moving aluminum sheet (2) is heated in the chamber (3)of the furnace (1) to the set peak metal temperature and soaked for anumber of seconds (t_(SOAK)) in the chamber (3) of the furnace (1) at atemperature in the range from the set peak metal temperature to thesoaking temperature T_(Soak) which is the predetermined desired minimumtemperature selected for annealing or solution heat treating. Bydefinition T_(Soak) is lower than peak metal temperature (T_(PMT)).

The moving or travelling aluminum sheet moves substantially horizontallyin a floating state through the elongated heat treatment chamber (3)over a length of typically at least about 20 meters, preferably over atleast 55 meters.

On leaving the exit portion (5) the moving aluminum sheet (2) is rapidlycooled in the cooling section (6) to below about 150° C., e.g. to aboutroom temperature.

Then the aluminum sheet (2) passes through a second looper accumulator(14) downstream of the furnace (3) and then proceeds to a shearingstation (18). The shearing station (18) cuts the heat treated aluminumsheet 2 into product sheets 20. For example, flying shears may cut theheat treated aluminum sheet 2 into product sheets 20.

FIG. 2 shows a schematic drawing of details of the first looperaccumulator (12). The first looper accumulator (12) has a series ofrollers defining a path that can be expanded or contracted toaccommodate a stoppage of the trailing end of the sheet (2) while thejoiner attaches a leading end of the roll (8) to the trailing end of thesheet (2). The second looper accumulator (14) would have the same orsimilar structure as the first looper accumulator (12) to accommodatethe sheet (2) while a portion of sheet (2) downstream of the secondlooper accumulator (14) is stopped or slowed at the shearing station(18) while the shearing station (18) cuts the heat treated aluminumsheet (2) into the product sheets (20) and being recoiled intoindividual coils.

The continuous heat-treatment furnace (1) is a continuous convectionfloating furnace arranged to heat the moving aluminum sheet to a setpeak metal temperature (T_(PMT)). The furnace (1) has a series ofcontiguous zones (10) in its chamber (3) arranged to heat the movingsheet (2) such that during normal operation at least one zone (10) heatsthe moving sheet (2) to the set peak metal temperature (T_(PMT)).

FIG. 3 shows a schematic drawing of details of a zone (10) of thecontinuous convection floating furnace (1). Each zone (10) has at leastone fan (30) above the aluminum alloy sheet (2) and at least one fan(32) below the aluminum alloy sheet (2). The fans (30), (32) blowrecirculated hot furnace air into respective upper and lower nozzleheader boxes (34), (36) which include and feed a respective plurality ofnozzles which blow the recirculated hot furnace air onto the sheet (2).The upper nozzle header box (34) blows the recirculated furnace airdownwardly onto the sheet (2) to heat and stabilize the moving aluminumalloy sheet 2. The lower nozzle header box (36) blows the recirculatedfurnace air upwardly onto the sheet 2 to heat, float and stabilize themoving aluminum alloy sheet (2) as it travels in direction of travel“T”.

Each zone (10) typically has at least one convection heater, for exampleburner (40), above the sheet (2) and at least one burner (42) below thesheet (2). Typically, the burners (40), (42) are fed by combustible gas,typically natural gas, lines (44), (46). Each zone (10) also has atleast one fresh air feed duct (50) above the sheet (2) and/or below thesheet (2) fed by fresh air intake conduit 51.

FIG. 3 illustrates gas firing burners with multiple air circulationfans. These burners are convective heaters. Preferably, gas firingburners with multiple air circulation fans perform the convectiveheating. However, various other convective heating means can be applied,e.g. resistance heating, in the continuous heat treatment furnace.

The moving aluminum sheet moves substantially horizontally through theelongated heat treatment chamber (3) of the continuous furnace over alength of at least about 20 meters, preferably at least 40 meters, andmore preferably of at least about 55 meters. A practical maximum lengthis about 125 meters, but the invention is not limited to this maximumlength.

FIG. 4 schematically shows a portion of the upper nozzle header box (34)to illustrate nozzles (35) which discharge into the space within theelongated heat treatment chamber (3) of the furnace.

FIG. 5 schematically shows a portion of the lower nozzle header box (36)to illustrate nozzles (37) which discharge into the space within theelongated heat treatment chamber (3).

The hot-recirculating furnace air nozzles throughout the furnace lengthheat the strip and keep it afloat on an air cushion. Thus, the strip istravelling in a floating state. Such a furnace is also known asconvection floating furnace. The elimination of mechanical contact atelevated temperature in the heat-treatment furnace translates into afault-free strip surface. The continuous heat-treatment furnace can bemodular in design; as such the furnace comprises several heating zonesthat use turbines (not shown) to generate an air channel consisting oftop and bottom airflows. The burners that heat the air preferably workwith combustion pre-heated air.

The moving sheet (2) enters the entry section (4) at V_(line,Set) atambient temperature and is gradually heated-up while travelling throughthe continuous heat-treatment furnace to a pre-set heat treatmenttemperature in the temperature range of 350° C. to 590° C., preferably450° C. to 590° C. In the continuous heat-treatment furnace the averageheat-up rate of the aluminum sheet is typically in a range of about10-15° C./sec for an about 1 mm thick sheet material. Depending on thestrip speed the strip temperature may reach the actual pre-set solutionheat treatment temperature only far into the second-half of the furnacelength or even near the end of the continuous heat-treatment furnace andit is actually soaked at the solution heat treatment temperature for avery short period of time, e.g. a few seconds. Thereafter the movingsheet leaves the heat-treatment furnace at the exit section (5) and isimmediately quenched in the cooling section (6).

The soaking temperature T_(Soak) is the predetermined desired minimumtemperature selected for annealing or solution heat treating. Bydefinition T_(Soak) is lower than peak metal temperature (T_(PMT)). Thesoaking time (t_(SOAK)) is the time the sheet is held at or aboveT_(Soak). The soaking time (t_(SOAK)) of the moving aluminum sheet is atleast one second, typically at least 5 seconds, more typically 5 to 30seconds, for example 10 seconds.

Generally, aluminum sheet speed (also known as line speed) through thefurnace is at least 3 meters/minute. Typical aluminum sheet speed isabout 20 to about 140 m/min.

Optionally, the quenched and moving aluminum sheet is stretched up toabout 0.7%, typically in a range of about 0.1% to 0.5%, by means oftension levelling. Preferably, the stretched and moving aluminum sheetis subsequently cleaned and provided with a coating, e.g., a passivationcoating, or otherwise processed.

Optionally, a pre-bake heat treatment heat-treats the stretched aluminumsheet having a passivation coating. The pre-bake treatment increases inparticular the paint-bake response of the AA6000-series aluminum sheetmaterial.

Furnace Control

In accordance with the invention this balance of properties and processeconomy has been improved by implementing a control mechanism andapparatus to maintain peak metal temperature in the furnace in the eventof a reduction in line speed.

To cope with different line speeds, the invention controls heattreatment furnace based on the actual line speed.

During continuous heat treating in the convection floating furnace tominimize the effect of line speed variations on peak metal temperature,the invention controls the speed of the top and bottom circulation fansand the zone temperature based upon line speed. The top and bottomcirculation fans in each zone circulate the air (controlling heattransfer) and floats the sheet through the furnace.

The zone temperatures are the furnace air temperatures of the zones ofthe convection floating furnace. The convection floating furnacetypically has more than one zone, for example 4 to 28 zones. The zonesare typically heated by burning natural gas or other combustible gas.

The fan speed and temperature have direct relation to the heat transfercoefficient (HTC) and line speed has a direct relation to the energyinput to the strip over the furnace length. Thus, a fan speed and zonetemperature control based on line speed is employed to have a peak metaltemperature (PMT) (and soak time) controlled system. Time in the furnacewill typically change when line speed changes, so maintaining the peakmetal temperature and minimizing soak time changes is beneficial.

The inventive method for continuously heat-treating aluminum alloy sheetat final thickness in a continuous heat treating furnace having an entrysection and an exit section, wherein the heat treating furnace is acontinuous convection floating furnace, comprising

continuously horizontally moving uncoiled aluminum alloy sheet in afloating state in a path along a direction of its length through aplurality of contiguous heat treatment zones of an elongated heattreatment chamber of the continuous heat-treating furnace arranged toheat the moving aluminum sheet to a set peak metal temperature (T_(PMT))in the temperature range of 350° C. to 590° C.;

wherein the contiguous heat treatment zones have independentlycontrollable convection heaters along the path for heating the aluminumalloy sheet and independently controllable fans blowing above and belowthe aluminum alloy sheet along the path for guiding the aluminum alloysheet along the path as the aluminum alloy sheet horizontally movesthrough the elongated heat treatment chamber,

wherein at least one said contiguous heat treatment zone is a peak metaltemperature zone which has a target aluminum alloy sheet temperaturewhich is the peak metal temperature of the aluminum alloy sheet in theelongated heat treatment chamber;

taking measurements representative of heat transfer to the aluminumalloy sheet as the aluminum alloy sheet moves through the elongated heattreatment chamber, the measurements including speed of the aluminumalloy sheet through the elongated heat treatment chamber, speed of thefans blowing above and below the aluminum alloy sheet, and furnace airtemperature, and optionally surface temperature of the sheet,

wherein said taking measurements comprises:

continuously measuring the line speed of horizontal movement of thealuminum alloy sheet through the furnace and generating a speed signalproportional to the actual measured line speed v_(line,act) of thealuminum alloy sheet through the furnace, wherein the line speed of thealuminum alloy sheet through the furnace has at a line speed set point(v_(line,Set)),

continuously measuring the speeds of the furnace fans above and belowthe aluminum alloy sheet in said peak metal temperature zone, whereinthe fan speeds of the fans above and below the aluminum alloy sheet haverespective fan speed set points (v_(fan,Set)), wherein the fan speed setpoint of the fans above the aluminum alloy sheet may be the same ordifferent from the fan speed set point of the fans below the aluminumalloy sheet;

continuously measuring the furnace air temperature in said peak metaltemperature zone, wherein the furnace air temperature in said peak metaltemperature zone has a zone air temperature set point (T_(zone,Set));and

optionally continuously measuring the surface temperature of the sheet,

wherein during normal operation the actual measured line speed, measuredfan speeds in said peak metal temperature zone, and measured furnace airtemperature in said peak metal temperature zone simultaneously arerespectively in the preset ranges for the line speed set point(v_(line,Set)), the fan speed set points (v_(fan,Set)), and the zone airtemperature set point (T_(zone,Set)) in said peak metal temperaturezone;

changing the fan speeds and the furnace air temperature in said peakmetal temperature zone, in response to the actual measured line speedv_(line,act) and the line speed set point v_(line,Set) according toequation (I) and equation (II):

$\begin{matrix}{v_{{fan},{act}} = {\left( \frac{v_{{line},{act}}}{v_{{line},{Set}}} \right)^{x} \times v_{{fan},{Set}}}} & (I) \\{T_{{zone},{act}} = {\left( \frac{v_{{line},{act}}}{v_{{line},{Set}}} \right)^{y} \times T_{{zone},{Set}}}} & ({II})\end{matrix}$

wherein:v_(line,act) is the actual measured line speed, for example in m/min,v_(line,Set) is the line speed set point, for example in m/min,v_(fan,act) is the actual fan speed in rpm in the peak metal temperaturezone,x is between 0.6 and 0.95, preferably 0.65 to 0.95, more preferably 0.75to 0.95,v_(fan,Set) is the fan speed set point in rpm in the peak metaltemperature zone,wherein the set point of each fan above the moving sheet may be the sameor different from the set point of each fan below the moving sheet,T_(zone,act) is the actual zone air temperature in the peak metaltemperature zone,T_(zone,Set) is the zone air temperature set point in the peak metaltemperature zone,y is between 0 and 0.2, preferably 0 and 0.1.

Preferably the measurements are taken for the first zone to achieve peakmetal temperature and all the contiguous heat treatment zones in thefurnace downstream of this peak metal temperature zone, more preferablyall the contiguous heat treatment zones in the furnace, and the fanspeeds and furnace air temperatures adjusted for the first zone toachieve peak metal temperature and all contiguous heat treatment zonesin the furnace downstream of this peak metal temperature zone, morepreferably all the contiguous heat treatment zones in the furnace.

Thus, the invention may control the sheet temperature by fan speed andfurnace air temperature compensation, not only in one or more peak metaltemperature zones, but also in additional zones of the furnace. Thus,taking measurements in the method of the invention may further comprise:

continuously measuring the speeds of the furnace fans above and belowthe aluminum alloy sheet in one or more additional zones of saidcontiguous zones, said additional zones being in addition to said peakmetal temperature zone, wherein for each additional zone the fan speedsof the fans above and below the aluminum alloy sheet have respective fanspeed set points (v_(fan,Set)), wherein the fan speed set point of eachfan above the aluminum alloy sheet may be the same or different from thefan speed set point of each fan below the aluminum alloy sheet;

continuously measuring the furnace air temperature in each additionalzone, wherein the furnace air temperature in each additional zone has arespective zone air temperature set point (T_(zone,Set)); and

optionally continuously measuring the surface temperature of the sheet,

wherein during normal operation the actual measured line speed, measuredfan speeds in each additional zone, and measured furnace air temperaturein each additional zone simultaneously are respectively in the presetranges for the line speed set point (v_(line,Set)), the fan speed setpoints (v_(fan,Set)), and the zone air temperature set point(T_(zone,Set)) in the additional zone;

respectively changing the fan speeds and the furnace air temperature ineach additional zone, in response to the actual measured line speedv_(line,act) and the line speed set point v_(line,Set) according toequation (I) and equation (II):

$\begin{matrix}{v_{{fan},{act}} = {\left( \frac{v_{{line},{act}}}{v_{{line},{Set}}} \right)^{x} \times v_{{fan},{Set}}}} & (I) \\{T_{{zone},{act}} = {\left( \frac{v_{{line},{act}}}{v_{{line},{Set}}} \right)^{y} \times T_{{zone},{Set}}}} & ({II})\end{matrix}$

wherein:v_(line,act) is the actual measured line speed, for example in m/min,v_(line,Set) is the line speed set point, for example in m/min,v_(fan,act) is the actual fan speed in rpm in the additional zone,x is between 0.6 and 0.95, preferably 0.65 to 0.95, more preferably 0.75to 0.95,v_(fan,Set) is the fan speed set point in rpm in the additional zone,wherein the set point of the fans above the moving sheet may be the sameor different from the set point of the fans below the moving sheet,T_(zone,act) is the actual zone air temperature in the respectiveadditional zone,T_(zone,Set) is the zone air temperature set point in the respectiveadditional zone,y is between 0 and 0.2, preferably 0 and 0.1.

Taking the measurements and adjusting fan speeds and furnace airtemperature in all the contiguous heat treatment zones involves a methodfor continuously heat treating aluminum alloy sheet at final thicknessin a continuous heat treating furnace having an entry section and anexit section, wherein the heat treating furnace is a continuousconvection floating furnace, comprising:

continuously horizontally moving uncoiled aluminum alloy sheetcontinuously moving in a floating state in a path along a direction ofits length through a plurality of contiguous heat treatment zones of anelongated heat treatment chamber of the continuous heat-treating furnacearranged to heat the moving aluminum sheet to a set peak metaltemperature (T_(PMT)) in the temperature range of 350° C. to 590° C.;

wherein the contiguous heat treatment zones have independentlycontrollable convection heaters along the path for heating the aluminumalloy sheet and independently controllable fans blowing above and belowthe aluminum alloy sheet along the path for guiding the aluminum alloysheet along the path as the aluminum alloy sheet horizontally movesthrough the elongated heat treatment chamber,

wherein at least one said contiguous heat treatment zone is a peak metaltemperature zone which has a target aluminum alloy sheet temperaturewhich is the peak metal temperature of the aluminum alloy sheet in theelongated heat treatment chamber;

taking measurements representative of heat transfer to the aluminumalloy sheet as the aluminum alloy sheet moves through the elongated heattreatment chamber, the measurements including speed of the aluminumalloy sheet through the elongated heat treatment chamber, speed of thefans blowing above and below the aluminum alloy sheet in each heattreatment zone, and furnace air temperature in each heat treatment zone,and optionally surface temperature of the sheet in each heat treatmentzone,

wherein said taking measurements comprises:

continuously measuring the line speed of horizontal movement of thealuminum alloy sheet through the furnace and generating a speed signalproportional to the measured line speed v_(line,act) of the aluminumalloy sheet through the furnace, wherein the line speed of the aluminumalloy sheet through the furnace has at a line speed set point(v_(line,Set)),

continuously measuring the speeds of the furnace fans above and belowthe aluminum alloy sheet in each heat treatment zone, wherein the fanspeeds of the fans above and below the aluminum alloy sheet haverespective fan speed set points (v_(fan,Set)) which lie withinrespective preset fan speed ranges, wherein the fan speed set point ofthe fans above the aluminum alloy sheet may be the same or differentfrom the fan speed set point of the fans below the aluminum alloy sheet;

continuously measuring the furnace air temperature in each heattreatment zone, such that the furnace air temperature in each heattreatment zone is at a respective zone air temperature set point(T_(zone,Set));

changing the fan speeds and the furnace air temperature in each heattreatment zone, in response to the actual line speed v_(line,act) andthe line speed set point v_(line,Set) according to equation (I) andequation (II) (per zone):

$\begin{matrix}{v_{{fan},{act}} = {\left( \frac{v_{{line},{act}}}{v_{{line},{Set}}} \right)^{x} \times v_{{fan},{Set}}}} & (I) \\{T_{{zone},{act}} = {\left( \frac{v_{{line},{act}}}{v_{{line},{Set}}} \right)^{y} \times T_{{zone},{Set}}}} & ({II})\end{matrix}$

wherein:v_(line,act) is the actual line speed, for example in m/min,v_(line,Set) is the line speed set point, for example in m/min,v_(fan,act) is the actual fan speed in rpm in the respective heattreatment zone,x is between 0.6 and 0.95, preferably 0.65 to 0.95, more preferably 0.75to 0.95, v_(fan,Set) is the fan speed set point in rpm, wherein the setpoint of the fans above the moving sheet may be the same or differentfrom the set point of the fans below the moving sheet in each respectiveheat treatment zone,T_(zone,act) is the actual zone air temperature in the respective heattreatment zoneT_(zone,Set) is the zone air temperature set point in the respectiveheat treatment zoney is between y is between 0 and 0.2, preferably 0 and 0.1.

In the invention, the zone air temperature set point (T_(zone,Set)) fora respective zone lies within a respective preset temperature range. Theline speed set point (v_(line,Set)) lies within a preset range for thespeed of the aluminum alloy sheet. The respective fan speed set points(v_(fan,Set)) lie within respective preset fan speed ranges. The zoneair temperature set point (T_(zone,Set)) for a respective zone lieswithin a respective preset temperature range.

The values for “x” and “y” can be empirically determined by a user byrunning computer simulation furnace models that calculate the effect ofvarying the line speed on peak metal temperature. Then to validate thesefindings actual furnace trials can be conducted varying the line speedand sampling the products from these trials. Changes in productproperties indicate the changes in PMT resulting from these line speedchanges.

Parameters x, y for an actual furnace are measured empirically bymeasuring actual aluminum sheet temperatures with sensors over multiplefurnace test runs at varying conditions of fan speed and zonetemperature and time. To estimate temperature during normal operationthe invention monitors and measures fan speed and zone furnace airtemperature to determine the heat transfer rate. In addition, theinvention monitors and measures line speed to determine the time themetal sheet is exposed to those conditions. Then an online computermodel based upon these measurements uses this information in heattransfer equations to calculate a peak metal temperature. Heat transferequations for convective heat transfer are known to those skilled in theart. See for example, Engineering Heat Transfer, M. M. Rathore, Jones &Bartlett Learning, 2d Ed. (2011); or Heat Transfer, J. P. Holman,McGraw-Hill, 10^(th) ed. (2010).

Typically, during furnace operation automated and continuous readingsare taken of fan rpm and zone furnace air temperature. This coupled withline speed are used in a mathematic model to calculate peak metaltemperature (PMT). For example, the model may be based on monitoring andmeasuring fan speed (rpm) and zone furnace air temperature to determinethe heat transfer rate along with line speed measurement to determinethe time the metal is exposed to those conditions

However, in the invention, the continuous measuring of the surfacetemperature of the sheet may also be accomplished by an infraredtemperature gun or other type of pyrometer, for instance, in the peakmetal temperature zone and/or one or more additional zones. Theinvention may employ a feedback loop with sheet surface temperaturemeasurement(s) instead of, or in addition to, using a model to determinethe metal temperature, e.g., peak metal temperature.

This control of fan speed and furnace air temperature according toequations (I) and (II) should be activated once the actual line speeddiffers from the line speed set point v_(line,Set) by more than asettable tolerance, e.g., 0 to 10% deviation from line speed set point,typically 1% to 5% deviation from line speed set point.

Fan speed parameter x is user selectable per furnace zone (in machineparameters) based on empirical data as explained above. However, ingeneral x is between 0.6 and 0.95, preferably 0.65 to 0.95, morepreferably 0.75 to 0.95, and may be the same or different for each zone.

There are fans above and below the sheet. The fans above may be at adifferent rpm than the fans below. However, the invention typicallycontrols/adjusts the actual fan speed v_(fan,act) rpm of both sets offans by the same ratio relative to their respective fan speed set pointv_(fan,Set).

Reducing the speed of the fans above and below the sheet to cope withline speed reductions is limited because safe floating of the sheet mustbe guaranteed all time. Thus, the formula above is limited to values forsafely floating the sheet. In particular, the reduction of fan speed islimited by: Minimum fan speeds to maintain flotation of the sheet abovefurnace components to avoid damage, such as scratching, to the sheet,Maximum soaking time; and Minimum heating rates. This is generally nomore than a 35% reduction in rpm, preferably no more than a 30%reduction in rpm.

Furnace air temperature parameter y is user selectable per furnace zone(in machine parameters) based on empirical data as explained above.However, in general y is between 0 and 0.2, preferably 0 and 0.1, morepreferably between 0.02 and 0.1, and may be the same or different foreach zone.

However, furnace air temperature should be reduced no more than 10° C.,preferably no more than 5° C., such that the inventive method can cooland heat up the furnace air in the same time period as the duration ofthe speed change.

Reducing the zone temperature to cope with line speed reductions islimited to a maximum deviation below recipe temperature set point. Everyzone (10) typically has at least one burner (two shown in FIG. 3) and atleast one fresh air cooling intake (two shown in FIG. 3). Thus, to raisetemperature the feed of combustible gas to the burner (40), (42) can beincreased and/or the influx of air from the fresh air cooling intake(51) to the fresh air feed duct (50) can be adjusted. For example, toraise the zone temperature the feed of combustible gas to the burner(40), (42) can be increased and/or the influx of air from the fresh aircooling intake can be decreased. Also for example, to lower the zonetemperature the feed of combustible gas to the burner (40), (42) can bedecreased and/or the influx of air from the fresh air cooling intake canbe increased.

The control of the peak metal temperature by means of fan speed andtemperature compensation maintains temperature typically within +/−5°C., preferably +/−3° C., more preferably +/−2° C. of target peak metaltemperature for all decelerating, constant, or accelerating speedconditions.

Control Scheme Details

FIG. 3 also schematically shows control scheme details of the zone (10)of the continuous convection floating furnace (1) that may be employedto implement this inventive method. Each zone (10) also has at least onefurnace air temperature sensor (52) in communication with a temperaturecontroller (54). 12. The furnace air temperature sensors in theinvention may be, for example, thermocouples.

The temperature controller (54) controlling the combustion gas feedvalves (56) and the fresh air intake valves (58). The line speed sensor(13) is also in communication with the temperature controller (54) andfan speed controllers (60), (62). Thus, when the line speed sensor (13)senses a change in the line speed of the sheet (2) it sends signals tothe fan speed controller (60) and to the temperature controller (54) tocontrol the respective speed of the fans (30), (32) and the furnace airtemperature according to according to the above listed equation (I) andequation (II).

When line speed sensor (13) senses a change in the line speed of thesheet (2) it sends at least one signal to the fan speed controller (60)to adjust the speeds of the respective fans (30), (32) according toequation (I). Also, when the line speed sensor (13) senses a change inthe line speed of the sheet (2) it sends signals to the furnace airtemperature controller (54) to control furnace air temperature accordingto equation (II). The furnace air temperature is controlled bycontrolling the air intake valves (58) which admit cooling air and thecombustion gas valves 56 which control feed of combustion gas to theburners (40), (42).

Additional Operations

If desired additional operations may be performed based on measurementsand control of line speed, furnace air temperature, and fan speeds.

The method may further include taking measurements representative ofheat transfer to the sheet as the sheet advances through the elongatedheat treatment chamber of the furnace which comprises measuring linespeed of the sheet through the elongated heat treatment chamber,measuring the fan speeds of fans above and below the moving sheet ineach contiguous heat treatment zone and measuring the furnace airtemperature in each contiguous heat treatment zone;

comparing the measured fan speeds with a preset fan speed range in theform of specified values;

comparing the measured furnace air temperature with a preset airtemperature range in the form of specified values;

providing a plurality of different independently-controllable burners atdifferent positions along the pathway, providing a plurality ofdifferent independently-controllable fans above and below the movingaluminum sheet at different positions along the pathway;

generating a real-time sheet temperature estimate of a single pointwherein the sheet temperature estimate is established as a function ofthe sheet line speed in the elongated heat treatment chamber, thefurnace air temperature in each contiguous heat treatment zone, and thefan speeds in each contiguous heat treatment zone (typically theinvention validates with the properties of the metal in times of linespeed flux);

adjusting the sheet temperature estimate in response to a change in atleast one of the sheet line speed, the furnace air temperature in atleast one said zone, and the fan speeds in at least one said contiguousheat treatment zone.

The sheet temperature estimates are sheet surface temperature estimates.

If desired the method may further include taking measurementsrepresentative of heat transfer to the aluminum alloy sheet as thealuminum alloy sheet moves through the elongated heat treatment chamberof the furnace, which comprises from time to time taking measurementsincluding speed of the aluminum alloy sheet through the elongated heattreatment chamber, speed of the fans above and below, and airtemperature in each of the contiguous heat treatment zones, generating areal-time sheet temperature estimate along the path wherein the sheettemperature estimate is established as a function of speed of thealuminum alloy sheet through the elongated heat treatment chamber, speedof the fans above and below, and air temperature in each contiguous heattreatment zone;

wherein said generating the real-time sheet temperature estimate alongthe path comprises estimating the aluminum alloy sheet temperature inthe peak metal temperature zone at the line speed set pointv_(line,Set), and the fan speed set point v_(fan,Set),

adjusting the sheet temperature estimate in response to a change in atleast one of the sheet speed, speed of the fans above and below, and airtemperature in each contiguous heat treatment zone;

comparing the sheet temperature estimate to a desired temperaturedistribution to determine any differences between the sheet temperatureestimate and the desired temperature distribution, wherein the desiredtemperature distribution includes a plurality of temperature setpointsfor the sheet along the pathway, each setpoint representing a targetvalue;

for each one of the burners and fans, regulating operation as a functionof the differences with a closed-loop, feedback controller; estimating,in real-time, a sheet temperature profile at a point along the pathwayfor the sheet based on the sheet temperature estimate and the length ofthe sheet, and visually displaying a real-time representation of thesheet temperature estimate and soak time, and, if desired, the sheetthickness profile and the length of the sheet.

Solution Heat Treating Process and Annealing Processes

A conventional process for producing aluminum alloy products in rolledform with solution heat treating includes the processing steps whereinan aluminum alloy body is cast, after which it is homogenized and thenhot rolled to an intermediate gauge. Next, the aluminum alloy body maybe cold rolled. Next it is solution heat treated and quenched, forexample by means of water such as water quenching or water sprayquenching. “Solution heat treating and quenching” and the like,generally referred to herein as “solutionizing”, means heating analuminum alloy body to a suitable temperature, generally above thesolvus temperature, holding at that temperature long enough to allowsoluble elements to enter into solid solution, and cooling rapidlyenough to hold the elements in solid solution.

The following are typical solution heat treating and annealing processeswhich may be performed in a furnace being controlled according to themethod of the invention.

To solution heat treat according to the method of the present inventionthe process includes continuously moving uncoiled heat treatablealuminum alloy sheet in the direction of its length horizontally throughthe continuous heat treatment furnace arranged to heat the movingaluminum sheet to heat the product at a heating rate in the range of 2to 200° C./sec to a solution heat treating temperature of typically 350°C. to 590° C., preferably 370° C. to 590° C., more preferably 460° C. to580° C., or more preferably 500° C. to 590° C., furthermore preferably480° C. to 580° C. and soak the sheet at this temperature for a soakingtime (t_(SOAK)). Then the sheet exiting the furnace is quenchedtypically by quench water at a cooling rate in the range of 10 to 500°C./sec to below a temperature of 150° C. Soaking temperature T_(Soak) isthe predetermined desired minimum temperature selected for the solutionheat treating. By definition T_(Soak) is lower than peak metaltemperature (T_(PMT)). The soaking time (t_(SOAK)) is the time the sheetis held at or above T_(Soak). The soaking time (t_(SOAK)) of the movingaluminum sheet is at least one second, for example at least 1 to at most100 sec, typically at least 5 seconds, more typically 5 to 30 seconds,e.g., 10 seconds.

When solution heat treating continuously moving uncoiled heat treatableAlMgSi aluminum alloy sheet (also known as AA6000-series alloy sheet) atfinal gauge the sheet is heated to and held (soaked) at solution heattreating temperature in a range of 500° C. to 590° C., preferably 520°C. to 580° C., to dissolve in particular Mg and Si. The furnace beingcontrolled according to the method of the invention. Then the sheetexiting the furnace is quenched typically by quench water at a coolingrate in the range of 10 to 500° C./sec to below a temperature of 150° C.Typical AA6000 alloys treatable according to the invention include 6005,6009, 6010, 6111, 6014, 6016, 6022, 6061, 6181, 6082, 6182, and variousothers. Solution heat treating of AA6000 series alloy involves recoveryin the upstream zone or zones in which the metal softens by rearrangingthe cold worked structure, recrystallization of the metal in the middlezone or zones, and grain growth in the metal in the downstream zone orzones of the furnace where the metal achieves and soaks at soakingtemperature during which the small recrystallized grains will grow tothe desired size.

When solution heat treating continuously moving uncoiled heat treatablealuminum alloy sheet of AA7000-series alloy at final gauge the sheet isheated to and held at solution heat treating temperature commonly in arange of about 430° C. to 560° C., preferably 450° C. to 560° C. Thesolid solution formed at high temperature may be retained in asupersaturated state by cooling with sufficient rapidity to restrict theprecipitation of the solute atoms as coarse, incoherent particles,typically by quench water at a cooling rate in the range of 10 to 500°C./sec to below a temperature of 150° C.

If desired, the AA7000-series aluminum alloy has a Cu-content of lessthan 0.25% and is one of the following AA7000-series aluminum alloys, asdefined by the Aluminum Association: 7003, 7004, 7204, 7005, 7108,7108A, 7015, 7017, 7018, 7019, 7019A, 7020, 7021, 7024, 7025, 7028,7030, 7031, 7033, 7035, 7035A, 7039, 7046, and 7046A. For AA7000-seriesalloys have no purposive addition of Cu, the solution heat treatmenttemperature should be at least 370° C. A preferred minimum temperatureis 400° C., more preferably 430° C., furthermore preferably 450° C., andmost preferably 470° C. The solution heat-treatment temperature shouldnot exceed 560° C. A preferred maximum temperature is 545° C., andpreferably not more than 530° C.

If desired, the AA7000-series aluminum alloy has a Cu-content of 0.25%or more and is one of the following AA7000-series aluminum alloys, asdefined by the Aluminum Association: 7009, 7010, 7012, 7014, 7016, 7116,7022, 7122, 7023, 7026, 7029, 7129, 7229, 7032, 7033, 7034, 7036, 7136,7037, 7040, 7140, 7041, 7049, 7049A, 7149, 7249, 7349, 7449, 7050,7050A, 7150, 7250, 7055, 7155, 7255, 7056, 7060, 7064, 7065, 7068, 7168,7075, 7175, 7475, 7076, 7178, 7278, 7278A, 7081, 7181, 7085, 7185, 7090,7093, 7095 and 7099. For AA7000-series alloys have a purposive additionof Cu, the solution heat treatment temperature should be at least 400°C. A preferred minimum temperature is 450° C., furthermore preferably460° C., and most preferably 470° C. The solution heat-treatmenttemperature should not exceed 560° C. A preferred maximum temperature is530° C., and preferably not more than 520° C.

The AA7000-series aluminum sheet may have Zn in the range of 2.0% to10.0%, and preferably in the range of 3.0% to 9.0%. The AA7000-seriesaluminum sheet may have Mg in the range of 1.0% to 3.0%. TheAA7000-series aluminum sheet may have Cu is the range of <0.25%,preferably Cu in the range of 0.25% to 3.5%.

The AA7000-series aluminum sheet may further comprise

Fe<0.5%, preferably <0.35%,

Si<0.5%, preferably <0.4%, and

one or more elements selected from the group consisting of:

-   -   Zr at most 0.5,    -   Ti at most 0.3,    -   Cr at most 0.4,    -   Sc at most 0.5,    -   Hf at most 0.3,    -   Mn at most 0.4,    -   V at most 0.4,    -   Ge at most 0.4,    -   Ag at most 0.5,    -   balance being aluminum and impurities.

Following solution heat treatment and cooling the AA7000-series aluminumsheet may have a recrystallized microstructure.

After solutionizing, the AA7000-series aluminum alloy body may beoptionally stretched a small amount (e.g., about 1-5%) for flatness,thermally treated (e.g. by natural ageing or artificial ageing) andoptionally subjected to final treatment practices (e.g. a formingoperation, paint-bake cycle in case of an automotive application).

Also, the method and apparatus of the invention can be applied to abroad range of heat-treatable aluminum alloys to be annealed or solutionheat treated. For example, depending on the actual alloy composition,the invention may be employed with lower solution heat treatmenttemperatures, e.g., in the range of 460° C. to 480° C.

The heat treatment process includes continuously moving uncoiled heattreatable aluminum alloy sheet in the direction of its lengthhorizontally through a continuous annealing furnace arranged to heat themoving aluminum sheet to heat the product to a temperature in the rangeof 370° C. to 560° C., more typically 480° C. to 560° C., for a durationof 1 to at most 100 sec. The moving aluminum sheet is rapidly cooled onleaving the furnace. The cooling is typically by quench water at acooling rate in the range of 10 to 500° C./sec to below a temperaturebelow 150° C.

For annealing AA5000 series aluminum alloy, holding the product atsuitable temperature in the typical multi zone continuous heat-treatingfurnace has the following three stages:

1) Recovery—in the upstream zone or zones in which the metal softens byrearranging the cold worked structure at a temperature in the range of350° C. to 450° C.,2) Recrystallization—in the middle zone or zones at an annealingtemperature in the range of 350° C. to 500° C., preferably 350° C. to450° C. wherein the metal is fully soft by removing dislocations andstarts forming small grains, and3) Grain growth—in the downstream zone or zones of the furnace thatachieves peak metal temperature (PMT) and soaks the metal at temperaturein the range of 450° C. to 590° C., preferably 460° C. to 550° C.wherein the small recrystallized grains will grow to the desired size.Then the sheet exits the furnace.

Typical AA5000 alloys treatable according to the present inventioninclude 5030, 5051, 5182, 5454, 5754, and various others.

The invention will now be illustrated with reference to a non-limitingexample according to the invention.

Example 1

FIG. 6 shows a chart illustrating the control of peak metal temperature(PMT) and line speed according to the present invention. This shows asaluminum alloy sheet material went through a speed reduction from 50 to35 m/min PMT (° C.) in a furnace the peak metal temperature varied anaverage+/−1° C. The fan speed and air temperature were adjustedaccording to the invention.

1. A method for continuously heating aluminum alloy sheet at finalthickness in a continuous heat-treating furnace having an entry sectionand an exit section, wherein the heat-treating furnace is a continuousconvection floating furnace, comprising continuously horizontally movinguncoiled aluminum alloy sheet in a floating state in a path along adirection of its length through a plurality of contiguous heat treatmentzones of an elongated heat treatment chamber of the continuousheat-treating furnace arranged to heat the moving aluminum sheet to aset peak metal temperature (T_(PMT)) in the temperature range of 350° C.to 590° C.; wherein the contiguous heat treatment zones haveindependently controllable convection heaters along the path for heatingthe aluminum alloy sheet and independently controllable fans blowingabove and below the aluminum alloy sheet along the path for guiding thealuminum alloy sheet along the path as the aluminum alloy sheethorizontally moves through the elongated heat treatment chamber, whereinat least one said contiguous heat treatment zone is a peak metaltemperature zone which has a target aluminum alloy sheet temperaturewhich is the peak metal temperature of the aluminum alloy sheet in theelongated heat treatment chamber; taking measurements representative ofheat transfer to the aluminum alloy sheet as the aluminum alloy sheetmoves through the elongated heat treatment chamber, the measurementsincluding speed of the aluminum alloy sheet through the elongated heattreatment chamber, speed of the fans blowing above and below thealuminum alloy sheet, and furnace air temperature, and optionallysurface temperature of the sheet, wherein said taking measurementscomprises: continuously measuring the line speed of horizontal movementof the aluminum alloy sheet through the furnace and generating a speedsignal proportional to the actual measured line speed v_(line,act) ofthe aluminum alloy sheet through the furnace, wherein the line speed ofthe aluminum alloy sheet through the furnace has at a line speed setpoint (v_(line,Set)), continuously measuring the speeds of the furnacefans above and below the aluminum alloy sheet in said peak metaltemperature zone, wherein the fan speeds of the fans above and below thealuminum alloy sheet have respective fan speed set points (v_(fan,Set)),wherein the fan speed set point of the fans above the aluminum alloysheet may be the same or different from the fan speed set point of thefans below the aluminum alloy sheet; continuously measuring the furnaceair temperature in said peak metal temperature zone, wherein the furnaceair temperature in said peak metal temperature zone has a zone airtemperature set point (T_(zone,Set)); and optionally continuouslymeasuring the surface temperature of the sheet, wherein during normaloperation the actual measured line speed, measured fan speeds in saidpeak metal temperature zone, and measured furnace air temperature insaid peak metal temperature zone simultaneously are respectively in thepreset ranges for the line speed set point (v_(line,Set)), the fan speedset points (v_(fan,Set)), and the zone air temperature set point(T_(zone,Set)) in said peak metal temperature zone; changing the fanspeeds and the furnace air temperature in said peak metal temperaturezone, in response to the actual measured line speed v_(line,act) and theline speed set point v_(line,Set) according to equation (I) and equation(II): $\begin{matrix}{v_{{fan},{act}} = {\left( \frac{v_{{line},{act}}}{v_{{line},{Set}}} \right)^{x} \times v_{{fan},{Set}}}} & (I) \\{T_{{zone},{act}} = {\left( \frac{v_{{line},{act}}}{v_{{line},{Set}}} \right)^{y} \times T_{{zone},{Set}}}} & ({II})\end{matrix}$ wherein: v_(line,act) is the actual measured line speed,for example in m/min, v_(line,Set) is the line speed set point, forexample in m/min, v_(fan,act) is the actual fan speed in rpm in the peakmetal temperature zone, x is between 0.6 and 0.95, v_(fan,Set) is thefan speed set point in rpm in the peak metal temperature zone, whereinthe set point of each fan above the moving sheet may be the same ordifferent from the set point of each fan below the moving sheet,T_(zone,act) is the actual zone air temperature in the peak metaltemperature zone, T_(zone,Set) is the zone air temperature set point inthe peak metal temperature zone, y is between 0 and 0.2.
 2. The methodof claim 1, wherein adjusting fan speed and furnace air temperature whenthe line speed is adjusted controls peak metal temperature in thefurnace peak metal temperature zone within +/−5° C. of set target peakmetal temperature (T_(PMT)).
 3. The method of claim 1, wherein actualfurnace air temperature is reduced relative to the furnace airtemperature set point no more than 10° C.
 4. The method of claim 1,which includes obtaining the at least one furnace air temperaturemeasurement with at least one temperature sensor within the furnace. 5.The method of claim 1, wherein the sheet moves along the pathway atleast three meters per minute.
 6. The method of claim 1, wherein thesheet is at peak metal temperature for 1 second to 100 seconds.
 7. Themethod of claim 1, comprising: at least one fan speed controllerconnected to a sensor measuring the line speed and to the fans in thepeak metal temperature zone, said fan speed controller controlling thefans to change respective fan speed from v_(fan,Set) to v_(fan,act) inthe event of a change of v_(line,act) to a value different fromv_(line,Set); at least one burner controller connected to the sensormeasuring the line speed of the aluminum sheet; and at least one furnaceair temperature controller connected to a temperature sensor measuringthe temperature of the furnace atmosphere said furnace air temperaturecontroller controlling the burners to change furnace air temperaturefrom T_(zone,Set) to T_(zone,act) in the event of a change v_(line,act)changing to a value different from v_(line,Set).
 8. The method of claim1, wherein the continuous furnace is heated by means of convectiveheating means.
 9. The method of claim 1, wherein the peak metaltemperature is 370° C. to 590° C.
 10. The method of claim 1, wherein thealuminum alloy sheet is heat-treatable AA6000 aluminum alloy sheet andthe set peak metal temperature (T_(PMT)) is in a range of 480° C. to590° C.
 11. The method of claim 1, wherein the aluminum alloy sheet isheat-treatable AA7000 aluminum alloy sheet and the set peak metaltemperature (T_(PMT)) is in a range of 430° C. to 560° C.
 12. The methodof claim 1, wherein the aluminum alloy sheet is heat-treatable AA7000aluminum alloy sheet and the set peak metal temperature (T_(PMT)) is inthe range of 460° C. to 480° C.
 13. The method of claim 1, wherein themethod anneals a non-heat-treatable AA5000-series aluminum alloy sheet,wherein continuously moving uncoiled non-heat-treatable AA5000-seriesaluminum alloy sheet moves in the direction of its length through thecontinuous heat-treatment furnace arranged to heat the moving aluminumsheet to peak metal temperature set peak metal temperature (T_(PMT)) inthe range of 350° C. to 560° C., and wherein the moving aluminum sheetis rapidly cooled from T_(PMT) to below about 150° C. on leaving thefurnace.
 14. The method of claim 1, comprising wherein said takingmeasurements representative of heat transfer to the sheet as the sheetadvances through the elongated heat treatment chamber comprisesmeasuring line speed of the sheet through the elongated heat treatmentchamber, measuring the fan speeds of fans above and below the movingsheet in each contiguous heat treatment zone and measuring the furnaceair temperature in each contiguous heat treatment zone; comparing themeasured fan speeds with a preset fan speed range in the form ofspecified values; comparing the measured furnace air temperature with apreset air temperature range in the form of specified values; providinga plurality of different independently-controllable burners at differentpositions along the pathway, providing a plurality of differentindependently-controllable fans above and below the moving aluminumsheet at different positions along the pathway; generating a real-timesheet temperature estimate along the pathway wherein the sheettemperature estimate is established as a function of the sheet linespeed in the elongated heat treatment chamber, the furnace airtemperature in each contiguous heat treatment zone, and the fan speed ineach contiguous heat treatment zone; adjusting the sheet temperatureestimate in response to a change in at least one of the sheet linespeed, the furnace air temperature in at least one said contiguous heattreatment zone, and the fan speeds in at least one said contiguous heattreatment zone.
 15. The method of claim 14, wherein the sheettemperature estimates are sheet surface temperature estimates.
 16. Themethod of claim 1, wherein said taking measurements representative ofheat transfer to the aluminum alloy sheet as the aluminum alloy sheetmoves through the elongated heat treatment chamber, comprises from timeto time taking measurements including speed of the aluminum alloy sheetthrough the elongated heat treatment chamber, speed of the fans aboveand below, and air temperature in each zone, generating a real-timesheet temperature estimate along the path wherein the sheet temperatureestimate is established as a function of speed of the aluminum alloysheet through the elongated heat treatment chamber, speed of the fansabove and below, and air temperature in each zone; wherein saidgenerating the real-time sheet temperature estimate along the pathcomprises estimating the aluminum alloy sheet temperature in the peakmetal temperature zone at the line speed set point v_(line,Set), and thefan speed set point v_(fan,Set), adjusting the sheet temperatureestimate in response to a change in at least one of the sheet speed,speed of the fans above and below, and air temperature in each zone;comparing the sheet temperature estimate to a desired temperaturedistribution to determine any differences between the sheet temperatureestimate and the desired temperature distribution, wherein the desiredtemperature distribution includes a plurality of temperature setpointsfor the sheet along the pathway, each setpoint representing a targetvalue; for each one of the burners and fans, regulating operation as afunction of the differences with a closed-loop, feedback controller;estimating, in real-time, a sheet temperature profile along the pathwayfor the sheet based on the sheet temperature estimate and the length ofthe sheet, and visually displaying a real-time representation of thesheet temperature estimate and soak time.
 17. The method of claim 1,wherein said taking measurements further comprises: continuouslymeasuring the speeds of the furnace fans above and below the aluminumalloy sheet in one or more additional zones of said contiguous zones,said additional zones being in addition to said peak metal temperaturezone, wherein for each additional zone the fan speeds of the fans aboveand below the aluminum alloy sheet have respective fan speed set points(v_(fan,Set)), wherein the fan speed set point of each fan above thealuminum alloy sheet may be the same or different from the fan speed setpoint of each fan below the aluminum alloy sheet; continuously measuringthe furnace air temperature in each additional zone, wherein the furnaceair temperature in each additional zone has a respective zone airtemperature set point (T_(zone,Set)); and optionally continuouslymeasuring the surface temperature of the sheet, wherein during normaloperation the actual measured line speed, measured fan speeds in eachadditional zone, and measured furnace air temperature in each additionalzone simultaneously are respectively in the preset ranges for the linespeed set point (v_(line,Set)), the fan speed set points (v_(fan,Set)),and the zone air temperature set point (T_(zone,Set)) in the additionalzone; respectively changing the fan speeds and the furnace airtemperature in each additional zone, in response to the actual measuredline speed v_(line,act) and the line speed set point v_(line,Set)according to equation (I) and equation (II): $\begin{matrix}{v_{{fan},{act}} = {\left( \frac{v_{{line},{act}}}{v_{{line},{Set}}} \right)^{x} \times v_{{fan},{Set}}}} & (I) \\{T_{{zone},{act}} = {\left( \frac{v_{{line},{act}}}{v_{{line},{Set}}} \right)^{y} \times T_{{zone},{Set}}}} & ({II})\end{matrix}$ wherein: v_(line,act) is the actual measured line speed,for example in m/min, v_(line,Set) is the line speed set point, forexample in m/min, v_(fan,act) is the actual fan speed in rpm in theadditional zone, x is between 0.6 and 0.95, v_(fan,Set) is the fan speedset point in rpm in the additional zone, wherein the set point of thefans above the moving sheet may be the same or different from the setpoint of the fans below the moving sheet, T_(zone,act) is the actualzone air temperature in the respective additional zone, T_(zone,Set) isthe zone air temperature set point in the respective additional zone, yis between 0 and 0.2.
 18. The method of claim 1, wherein x is between0.65 to 0.95 and y is between 0 and 0.1.
 19. The method of claim 18,wherein x is between 0.75 to 0.95.
 20. The method of claim 17, wherein yis between 0 and 0.1.